This invention relates to signal processing equipment for radiation imaging apparatus such as an Anger-type gamma camera disclosed in U.S. Pat. No. 3,011,057.
A conventional Anger-type gamma camera comprises a scintillation crystal responsive to radiation stimuli for producing light events at spatial locations corresponding to the locations at which the stimuli interact with the crystal, and a plurality of photodetectors arranged in a predetermined array with respect to the crystal for viewing light events therein and producing output signals in reponse thereto.
Associated with a gamma camera is signal processing equipment including means responsive to the output signals for generating an energy signal representative of the total energy of the light event producing such signals, and coordinate computation circuitry, responsive to the output signals of the photodetectors of the camera produced in response to the occurrence of a light event in the crystal, for computing the spatial coordinates of the light event provided the energy signal lies within a predetermined energy window.
Gamma cameras of the type described above are well known in the art and are in wide use in the nuclear medicine field for the purpose of obtaining pictures of the distribution of radioactivity in an object under investigation such as an organ of the human body containing a diagnostic quantity of a radioactive tracer. Depending upon the type of tracer utilized, all direct radiation stimuli originating in the organ will have a predetermined energy level, as for example 120 Kev. In addition to direct radiation incident on the scintillation crystal at a known energy level due to the tracer present in the organ under study, other types of radiation stimuli are also incident on the crystal and interact therewith. For example, background radiation, as from cosmic radiation, will always be present and will interact with the crystal. In addition, radiation stimuli resulting from the scattering of direct radiation stimuli originating from the tracer will also be incident on the scintillation crystal. These scattered radiation stimuli include the so called Compton products resulting from the interaction of direct radiation stimuli with soft tissue in the human body.
Since the direct radiation stimuli arriving in the crystal have a predetermined energy level, the scattered radiation stimuli can be discriminated against by passing the energy signal produced by the signal processing equipment associated with a camera through a single channel analyzer having an energy window comprehending the energy level associated with the tracer being utilized. Because of statistical variations in the number of photons created in each light event due to the interaction of a radiation stimulus with the scintillation crystal, the size of the energy window may be as large as 20% of the known energy level associated with the tracer in order to provide for the detection of a usable number of events. However, it is usually the case that the size of the window is so large that scattered radiation stimuli interacting with the crystal are detected and considered as if such events were produced by direct radiation stimuli. This situation is partially responsible for the spatial non-homogenity of gamma cameras; and it is an object of the present invention to provide new and improved signal processing equipment for use with such cameras wherein compensation is provided for spatial non-homogenity.